The present disclosure concerns improvements to gas turbine systems used in mechanical drive applications. In particular, but not exclusively the disclosure concerns gas turbine systems for driving compressors, e.g. compressors for refrigerant fluids in liquefied natural gas facilities.
The disclosure further concerns improvements in methods for operating a system comprising a gas turbine and a load, e.g. a compressor for LNG, or oil and gas applications.
Liquefied Natural Gas (LNG) results from a liquefaction process, in which the natural gas is cooled using one or more refrigeration cycles in a cascade arrangement, until it becomes liquid. Natural gas is often liquefied for storage or transportation purposes, e.g. if pipeline transportation is not possible or economically unfeasible.
Cooling of the natural gas is performed using closed or opened refrigeration cycles. A refrigerant is processed in a compressor or compressors, condensed and expanded. The expanded, chilled refrigerant is used to remove heat from the natural gas flowing in a heat exchanger.
Refrigerant compressors in LNG, pipeline applications or other applications in the oil and gas industry, are often driven by gas turbines. The gas turbine power availability (i.e. the power available on the turbine power shaft) is dependent upon ambient conditions, e.g. air temperature, and other factors, such as ageing. The turbine power availability increases with decreasing temperatures and, conversely, decreases with increasing temperatures. This causes power availability fluctuations both in the 24 hours as well as during the year, due to daily and seasonal temperature fluctuations.
It has been suggested to provide an electric motor in combination with a gas turbine to drive a load, comprised of e.g. one or more compressors. The electric motor is operated to supplement mechanical power to the compressor or compressors to maintain the overall mechanical power on the compressor shaft constant, when power availability of the turbine decreases and/or to increase the total mechanical power used to drive the load. This function of the electric motor is referred to as helper duty. The same electric motor is usually used also as a starter motor, to accelerate the string formed by the gas turbine and the compressor or compressors from zero to the rated speed.
When an excess mechanical power is generated by the turbine, e.g. if the ambient temperature drops below the design temperature and consequent increase in power availability of the turbine, the excessive mechanical power generated by the gas turbine is converted into electric energy, using the electric helper motor as a generator.
FIG. 1 illustrates a gas turbine and compressor arrangement with a helper/starter/generator machine, typically used in an LNG facility. The gas turbine 1 is connected via a common shaft line 3 to an electric motor/generator 5. The shaft line can be comprised of a plurality of shaft portions 3A, 3B, 3C, 3D. Reference number 4 designates a rigid coupling arranged between the gas turbine and the electric motor/generator 5. A further flexible coupling 6 is arranged between the electric motor/generator 5 and a load 7, e.g. a compressor. The electric motor/generator 5 has a drive-through capability, i.e. is designed to allow the mechanical power generated by the gas turbine 1 to be transmitted through the motor/generator 5 to the compressor 7. The drive-through capability must be equal to or greater than the gas turbine output power. The electric motor/generator 5 is connected to an electric power grid G through a frequency converter 11.
The electric motor/generator 5 is used as a starter to accelerate the gas turbine 1 from zero speed to full speed. Since the electric motor/generator 5 is located on the common shaft line 3, when performing the starter function, the motor/generator 5 also accelerates the entire compression string, i.e. the compressor or compressors 7. This requires the electric motor/generator 5 to be sufficiently powerful to accelerate the mass of all the rotational machines connected to the common shaft line 3, and also to overcome the aerodynamic load of the compressor or compressors 7, since during start-up the working fluid present in the compressor or compressors 7 starts flowing and the pressure thereof increases.
In other known natural gas liquefaction facilities the electric motor/generator is connected at one end of the compressor or compressors and the gas turbine is arranged at the opposite end of the compressors. The compressor or compressors is thus located between the gas turbine and the electric helper/generator. When the compressor is a vertically-split compressor, the electric motor/generator must be removed if the compressor requires maintenance. Moreover, in these known configurations, a dedicated starter for the gas turbine is provided on the cold end side of the gas turbine.